He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
Bakugo would likely be furious. He'd feel betrayed and would probably be the first to attack Deku, yelling at him about how he's thrown away everything they've worked for. Uraraka might be more empathetic. She'd try to understand what drove Deku to become a villain and hope there's a way to bring him back to the side of good.
Well, Iida would probably have a very proper reaction. He might give Bakugou a stern talk about treating Deku right and being respectful on the date. Mina would be super excited and would start gossiping about it right away. She'd be all over Deku, asking him what he was going to wear and things like that. And Kaminari would probably make some silly jokes about it, like saying Bakugou was finally 'going soft' for Deku.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
In a Naruto world fanfic, Killua would bring a whole new dynamic. He has a dark past as an assassin which could either make him a feared or respected figure. His family's training has made him physically and mentally tough. In battles, he could use his electricity - based attacks in creative ways against ninjas. But he would have to learn about things like jutsus and the ninja villages. Also, his relationship with Gon in the HxH world was important for his character development, and in the Naruto world, he might find a similar bond with a fellow shinobi, which could change his perspective on things.
His friends would be really shocked at first. Uraraka might be the first to try and comfort him, seeing as she has a kind heart. Bakugo, on the other hand, would probably be both confused and a bit angry at the situation. All - Might would likely feel a sense of responsibility to protect Deku even more than before. The teachers at UA would probably start researching how to reverse the transformation immediately.
Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative crossovers. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.